JP2021113359A - ANTICORROSION OF Al/Zn BASED COATING - Google Patents

Abstract

To reduce red rust of an Al/Zn coated steel strip under an environment of acid rain or pollution and corrosion at an end of the cross section of the strip under a marine environment.SOLUTION: There are selected an AL-Zn-Si-Mg alloy coating and its components (Mg and Si in principle), where the alloy coating has an OT:SDAS ratio of 0.5:1 or more (OT indicates a coating thickness on the surface of a strip; and SDAS indicates a measured value of a second dendrite arm space of Al-rich alpha-phase dendrite of the coating). The AL-Zn-Si-Mg alloy coating is formed on a steel strip by controlling solidification (cooling rate in principle) and forming Mg2Si phase particles of a specific form in an inter-dendrite channel. The invention relates to formation of the AL-Zn-Si-Mg alloy coating and a metal strip having the alloy coating.SELECTED DRAWING: None

Description

The present invention relates to the production of a product having an alloy coating containing aluminum and zinc as the main components of the alloy (hereinafter, referred to as "Al / Zn-based alloy coated product").

The term "Al / Zn-based alloy coated product" is understood to include products in the form of strips, tubes and structural sections having an Al / Zn-based alloy coating on at least a portion of the surface of the product.

The present invention includes, but is not limited to, Al / Zn-based alloy coated products in the form of metal (eg, steel) strips having an Al / Zn-based alloy coating on at least one surface of the strip, and Al / Zn-based alloy coatings. Regarding products obtained from strips.

The Al / Zn-based alloy coated metal strip may be a strip coated with an inorganic and / or organic compound for protective, aesthetic or other reasons.

The present invention relates to an Al / Zn-based alloy coated steel strip having a coating of one or more elements other than Al and Zn, for example Mg and Si, in an amount equal to or greater than a trace amount, particularly but not exclusively.

The present invention is more particularly but not limited to Mg containing 20-95% Al, less than 5% Si, less than 10 Mg and the remaining Zn and a small amount of other elements, typically less than 0.5% of each element. And for Al / Zn-based alloy coated steel strips with a coating of Al / Zn-based alloy containing Si (all% are by weight%). Unless otherwise specified, all with respect to the percentage of elements herein is understood to be percent by weight.

A thin (ie, 2-100 μm thick) Al / Zn-based alloy coating is often formed on the surface of the steel strip to provide corrosion protection.

Al / Zn-based alloy coatings are generally, but not limited to, alloys of Al and Zn, as well as Mg, Si, Fe, Mn, Ni, Sn and small amounts of other elements such as V, Sc, Ca, Sb. Is.

The Al / Zn-based alloy coating is generally, but not limited to, formed on the steel strip by thermal immersion coating by passing the strip through a molten alloy bath. Steel strips are typically, but not limited to, heated prior to immersion to facilitate the bonding of the alloy to the strips. As the strip emerges from the melting bath, the alloy solidifies on the strip to form a solidified alloy coating.

The Al / Zn-based alloy coating typically has a microstructure in the form of dendrites, mainly with a Zn-rich eutectic phase mixture in the region between the Al-rich alpha phase and the dendrites. When the solidification rate of the melt film is suitably controlled (eg, as in US Pat. No. 3,782,909 (insert this description herein)), the Al-rich alpha phase solidifies as sufficiently fine dendrites. It is placed in an interdendrite-like region to form a continuous network of channels, where the Zn-rich eutectic phase mixture solidifies.

The performance of these coatings is based on a combination of (a) first steel-based sacrificial protection with Zn-rich interdendritic eutectic phase mixing and (b) barrier protection with supporting Al-rich alpha phase dendrites. Zn-rich interdendrite phase mixing preferentially corrodes to provide sacrificial protection for steel substrates, and in some environments the Al-rich alpha phase is suitable for sacrificial protection for steel substrates when Zn-rich interdendrite phase mixing is eliminated. We will continue to provide level stuff and also provide barrier protection.

However, the levels of barrier protection and sacrificial protection provided by Al-rich alpha phase dendrites are inadequate and may result in poor performance of the coated steel strip. In such cases, there are three cases:

1. 1. An "acid rain" or "polluting" environment containing high concentrations of nitrogen oxides and sulfur oxides.
2. Under paint film in a marine environment.
3. 3. A cut surface or other area where the metal coating is damaged and the steel substrate is exposed to the marine environment.

As an example, Applicants have stated that the Al / Zn-based alloy coating on a steel strip is particularly thin (ie, a coating with a total coverage of 200 g / m ² or less, typically 150 g / m ² or less, which is a steel strip. When coated on each surface of the above with an equal thickness, one side has 100 g / m ² or less, typically 75 g / m ² or less), the microstructure is such that the coating has a standard cooling rate, typically. It was found that when formed at 11 ° C./s to 100 ° C./s, it tends to be columnar or bamboo-like extending from the steel strip to the coating surface. This microstructure includes (a) Al-rich alpha phase dendrites and (b) Zn-rich eutectic phase mixing formed as many separate columnar channels extending directly from the steel strip to the coating surface.

The applicant also found the following: That is, when a steel strip with a thin Al / Zn-based alloy coating having a columnar microstructure is exposed to a low pH environment, commonly referred to as an "acid rain" environment, or a high, commonly referred to as a "corrosion" environment. When exposed to environments with concentrations of sulfur dioxide and nitrogen oxide, the Zn-rich interdrenated eutectic phase mixture is quickly attacked and the columnar channels of this phase mixture extending directly from the steel strip to the coating surface are steel. Acts as a direct corrosion path to the strip. In the presence of such a direct corrosion path from the coating surface to the steel strip, the steel strip is prone to corrosion and corrosion products (iron oxides) move freely over the coating surface, resulting in "red rust styling". ) ”Appears. Red rust deteriorates the aesthetic appearance of coated steel products and also deteriorates product performance. For example, red rust reduces the thermal efficiency of coated steel products used as roofing materials.

The thin Al / Zn-based alloy coating is scratched, cracked or otherwise damaged to expose the steel strips, resulting in columnar red rust when exposed to "acid rain" or "contaminated" environments. The applicant also found that it could occur in the absence of a bamboo structure.

It has also been found that in "acid rain" and "polluted" environments, the Al-rich alpha phase cannot sacrificially protect the steel strip.

An "acid rain" environment, as used herein, means an environment in which the concentration formed on rain and / or coated steel strips is pH 5.6 or less. Illustratively, a "polluted" environment is defined in ISO 9223 as a P2 or P3 category, although not limited.

Also, in a marine environment, if Al-rich alpha phase dendrites are generally considered to provide excellent sacrificial protection for steel strips, this ability is a micro-environment under the paint film formed on the metallic coated steel strips. Decreases due to changes in.

The above statement should not be taken as an acceptance of common sense in Australia and elsewhere.

Applicants have found that red rust on Al / Zn-based alloy coated steel strips in an "acidic rain" or "contaminated" environment forms a coating as an Al-Zn-Si-Mg alloy coating, and the OT: SDAS ratio of the coating (in the ratio, middle, OT is the film thickness on the strip surface and SDAS is the second dendrite arm space of the Al-rich alpha phase dendrite in the coating.) Can be prevented or reduced by ensuring that it has 0.5: 1 or greater. I found.

The above-mentioned "overlay thickness" is understood to mean (total thickness of the coating film on the strip)-(thickness of the intermediate metal alloy layer of the coating film), and the intermediate metal alloy layer covers the coating film. It is a layer of an adjacent Al-Fe-Si-Zn4 elemental intermediate metal phase of a steel substrate formed by the reaction between the melt film and the steel substrate when formed into a strip.

That is, the present invention is a method of forming a coating of an anticorrosion Al-Zn-Si-Mg alloy suitable for an "acid rain" or "contamination" environment on a metal strip, typically a steel strip. a) A step of passing a metal strip through a melting bath of an Al-Zn-Si-Mg alloy to form an alloy coating on both sides or one side of the strip, (b) solidifying the coating on the strip and extending from the metal strip. Including the step of forming a solid film having a microstructure having _{Mg 2} Si phase particles in an interdrenated channel containing an interdrenated light channel of a mixture of Al-rich alpha phase dendrite and Zn-rich co-crystal phase. The method comprises control steps (a) and (b) and has an OT: SDAS ratio (in the ratio, OT is the film thickness and SDAS is the second dendrite arm space of the Al-rich alpha phase dendrite of the coating). Provided is a method for forming an anticorrosion Al—Zn—Si—Mg alloy coating on a metal strip, which comprises forming a solid coating having 0.5: 1 or more.

It is a graph of edge undercut and Mg concentration in the example of the Al-Zn-Si-Mg alloy coating according to the present invention on the test sample in the marine environment. ~ It is a photograph of a test panel and an image of a corroded front showing the performance improvement of an example of an Al-Zn-Si-Mg alloy coating according to the present invention in a marine environment. It is a photograph of a accelerated test panel in the laboratory showing the improvement of surface weathering and sacrifice protection for metal coated steel strips according to the present invention. ~ It is a photograph of a test panel showing the performance improvement of an example of an Al-Zn-Si-Mg alloy coating on a steel strip according to the present invention in an "acid rain" or "contaminated" environment. It is a top view of the scanning electron microscope image of the Al-Zn-Si-Mg alloy coating by this invention, and _{shows the morphology of Mg 2} Si phase particles in the microstructure shown in the image. _{FIG. 2 is} a network three-dimensional image of the form of Mg 2 Si phase particles in the Al—Zn—Si—Mg alloy coating of FIG.

The term "Zn-rich eutectic phase mixing" is understood to mean mixing the product of the eutectic reaction with a mixture of Zn-rich β phase and Mg: Zn compound phase, for example MgZn ₂ .

The present invention is also a metal strip having an Al-Zn-Si-Mg alloy coating on one or both sides suitable for acidic rain, contaminated environments, etc., and the coating extends from the metal strip to form an Al-rich alpha. It contains a phase dendrite and a Zn-rich eutectic phase mixed interdrenite channel _{, has a microstructure with Mg 2} Si phase particles in the interdrenated channel, and the coating has an OT: SDAS ratio (in ratio, OT). Is the film thickness, and SDAS is the second dendrite arm space of the Al-rich alpha phase dendrite of the coating.) Has an Al—Zn—Si—Mg alloy coating characterized by having 0.5: 1 or more. Provide metal strips.

In this case, the coatings are on both sides of the strip and the film thickness on each surface may be the same or different, depending on the requirements of the coating strips. In any case, the present invention requires that the OT: SDAS ratio be greater than 0.5: 1 for each surface coating.

The OT: SDAS ratio may be greater than 1: 1.
The OT: SDAS ratio may be greater than 2: 1.
The coating may be a thin film coating.
The SDAS of the Al-rich alpha phase dendrite in the coating is greater than 3 μm and less than 20 μm.

As used herein, a "thin film" coating on a metal (eg, steel) strip means that the total coating mass on either side of the strip is 200 g / m ² or less, which is one side of the steel strip. Equivalent to 100 g / m ² or less (of course not in all cases).

The coating amount of the coating film may be larger than 3 μm.
The coating amount of the coating film may be smaller than 20 μm.
The coating amount of the coating film may be smaller than 30 μm.
The coating amount of the coating film may be 5 to 20 μm.

The Al-Zn-Si-Mg alloy has 20-95% Al, up to 5% Si, up to 10% Mg and the remaining Zn, and has a small amount of other elements, typically less than 0.5% each of the other elements. May be good.

The Al-Zn-Si-Mg alloy may contain 40 to 65% Al.
The Al-Zn-Si-Mg alloy may contain 45 to 60% Al.
The Al-Zn-Si-Mg alloy may contain 35 to 50% Zn.
The Al—Zn—Si—Mg alloy may contain 39 to 48% Zn.
The Al-Zn-Si-Mg alloy may contain 1 to 3% of Si.
The Al-Zn-Si-Mg alloy may contain 1.3 to 2.5% of Si.
The Al-Zn-Si-Mg alloy may contain Mg in an amount of 5% or less.
The Al-Zn-Si-Mg alloy may contain 3% or less of Mg.
The Al-Zn-Si-Mg alloy may contain 1% or less of Mg.
The Al-Zn-Si-Mg alloy may contain 1.2 to 2.8% of Mg.
The Al-Zn-Si-Mg alloy may contain 1.5 to 2.5% of Mg.
The Al-Zn-Si-Mg alloy may contain 1.7 to 2.3% of Mg.
The metal strip may be a steel strip.

In addition to or in cases where the OT: SDAS ratio cannot be maintained and the coating has an OT: SDAS ratio of 0.5: 1 or less, the applicant is required to perform red rust and red rust in an "acid rain" or "contaminated" environment. Rust on fracture surfaces in marine environments can be prevented or reduced in thin Al-Zn-Zi-Mg alloy coatings on steel strips by selecting the composition of the coating alloy (mainly Mg and Si) and controlling the microstructure of the coating. I also understood that.

Selection of the above composition and control of the microstructure are particularly useful for thin film coatings and / or coatings having an OT: SDAS ratio of 0.5: 1 or less, but are not limited to these coatings, and are thick coatings. And / or it can also be applied to a film having an OT: SDAS ratio of 0.5: 1 or more.

Applicants also found that red rust in "acid rain" or "contamination" environments and rust on fracture surfaces in marine environments in Al / Zn-based coatings.
1. 1. Block corrosion to steel strips along Zn-rich interdendritic channels and / or 2. Activating the Al-rich alpha phase in these environments so that it can sacrificially protect the steel strip,
It was found that it can be suppressed or reduced by.

In both cases, the present invention is a metal strip having a coating of Al—Zn—Si—Mg alloy on one or both sides suitable for acidic rain, contaminated environments, etc., the coating extending from the metal strip and causing Al. Al-Zn-Si-Mg, which contains an interdentite channel of a rich alpha phase dendrite and a Zn-rich eutectic phase mixture, _{and has a microstructure having Mg 2 Si phase particles in the interdentite-like channel.} A metal strip having an alloy coating is provided.

In the above, the "particle" is the Mg ₂ Si phase and is understood to indicate the physical formation of a precipitate of this phase in the microstructure. Therefore, the "particles" are formed by precipitation from the solution when the coating solidifies and are not added separately to the composition.

1. 1. Blocking According to the present invention, it is a method of forming a coating of an anticorrosion Al-Zn-Si-Mg alloy suitable for an "acid rain" or "contamination" environment on a metal strip, typically a steel strip. ,
(A) A step of passing a metal strip through a melting bath of an Al-Zn-Si-Mg alloy to form an alloy film on both sides or one side of the strip.
(B) A micro that solidifies the coating on the strip, extends from the metal strip, contains an Al-rich alpha phase dendrite and a Zn-rich eutectic phase mixed dendrite channel, _{and has an Mg 2} Si phase in the interdentite channel. The process of forming a solid coating with a structure,
_{The Mg 2} Si phase in the interdendritic channel of the solidified coating, the method of which selects the Mg and Si concentrations and controls the cooling rate in step (b) to block corrosion along the interdedrite channel. Provided is a method for forming an anticorrosive Al-Zn-Si-Mg alloy film on a metal strip, which comprises forming particles of.

In an Al / Zn-based coating with a dendrite-like structure, Si exists as particles with flake-like morphology and does not corrode itself, but it does not fill the dendrite-like channels and from interdendritic corrosion to steel strips. Do not block the channel. Applicants have stated that Mg added to a Si-containing Al / Zn base coating, when combined with _{Si, forms Mg 2} Si phase particles in an interdendrite-like channel between the arms of an Al-rich alpha phase dendrite, which is appropriate. Large and shaped, it blocks what should otherwise be a direct corrosion path to the steel strip and helps isolate the steel substrate cathode beneath it. Particles of appropriate size and morphology are formed by controlling solidification, i.e. controlling the cooling rate of the coating.

In particular, the applicant set the cooling rate (CR) to 170-4.5 CT or less at the time of film solidification (where CR is the cooling rate at ° C./sec and CT is the film thickness (micrometer) on the strip surface. There is.) Should be maintained.

The morphology of Mg ₂ Si phase particles of appropriate size is the morphology of a "Chinese script" when viewed in a plane, and the morphology of petals in a three-dimensional image. Examples of this form are shown in FIGS. 12 and 13 and will be further described below.

The petals of Mg ₂ Si particles may have a thickness of less than 8 μm.
The petals of Mg ₂ Si particles may have a thickness of less than 5 μm.
The petals of Mg ₂ Si particles may have a thickness of 0.5 to 2.5 μm.

The concentration of Mg may be selected to be 0.5% or more. Below this concentration, there are not _{enough Mg 2} Si phase particles to fill and block the interdendritic channels.

The concentration of Mg may be selected to be 3% or less. Above this concentration results in _{large Mg 2} Si particles with cube-shaped morphology that are ineffective in blocking interdendritic corrosion.

In particular, the Al-Zn-Si-Mg alloy may contain 1% or more of Mg.

In order to coat with a Si concentration of 0.5 to 2%, _{the volume ratio of the interdendritic Mg 2} Si phase may be 50% or more as compared with other Si-containing phases.

_{The volume ratio of the interdendritic Mg 2} Si phase may be 80% or more as compared with other Si-containing phases.

_{The proportion of interdendrite Mg 2} Si present in the lower two-thirds of the film thickness is greater than or equal to 70% of the total integral ratio of _{the Mg 2} Si phase in the film to provide good blocking of the interdendrite channels. May be.

The proportion of interded light channels "blocked" by the Mg ₂ Si phase may be 60% or more, typically 70% or more, of the total number of channels.

Applicants also apply the improved protection possible in the present invention to the microstructure range from coarse dendrite structures with an OT: SDAS ratio of 0.5: 1 to fine dendrite structures with an OT: SDAS ratio of 6: 1. Will be done.

Corrosion along these conductions in general, and red rust through these conductions, especially in "acid rain" and "contaminated" environments, is slowed down.

In Al / Zn alloy coatings, corrosion along the interdendrite channels results in an increase in cooling rate during solidification, thereby reducing the SDAS of the coating, as described in US Pat. No. 3,782,909. The size of the channel may be reduced to limit it. However, although this may slow the surface corrosion of the coating (as often determined by mass loss testing), it limits the presence of zinc-rich phase mixes that provide sacrificial protection for steel substrates. Therefore, corrosion of the steel substrate occurs more easily.

2. Activation of the Alpha Phase The present invention also provides a method for forming a coating of an anticorrosion Al—Zn—Si—Mg alloy suitable for “acid rain” and “contamination” environments on metal strips, typically steel strips. And
(A) A step of passing a metal strip through a melting bath of an Al-Zn-Si-Mg alloy to form an alloy film on both sides or one side of the strip.
(B) A micro that solidifies the coating on the strip, extends from the metal strip, contains an Al-rich alpha phase dendrite and a Zn-rich eutectic phase mixed dendrite channel, _{and has an Mg 2} Si phase in the interdentite channel. The process of forming a solid coating with a structure,
_{Mg 2} with a size range, morphology and specific distribution that activates the Al-rich alpha phase and provides sacrificial protection by further selecting Mg and Si concentrations and controlling the cooling rate in step (b). Provided is a method for forming a layer of an anticorrosion Al—Zn—Si—Mg alloy, which comprises forming Si phase particles in an interdendrite-like channel in a solidified film.

In particular, the applicant has found that the Mg ₂ Si phase itself is reactive and can easily corrode. However, Applicants have also _{found conditions that passivate the Mg 2} Si phase, block and promote channels, and improve the activity of the Al-rich alpha phase in sacrificial protection of steel strips.

In particular, the applicant may add appropriate Mg and Si concentrations to the Al / Zn-based alloy coating composition and choose a cooling rate to solidify the alloy composition coating on the steel strip, resulting in interdendrites. _{The Mg 2} Si phase will be formed at a suitable dispersion and location in the channel, which activates the Al-rich alpha phase and steel in certain marine environments and in "acid rain" and "contamination" environments. Provides sacrificial protection.

Activation of the Al-rich alpha phase allows the application of fine dendrite structures when the steel substrate is exposed, without the loss resulting from sacrificial protection at the cut edges or other regions.

The selection and cooling rate of Mg and Si concentrations follows these parameters described in the "blocking" section.

In particular, in the case of cooling rate, the applicant stated that the cooling rate CR during film solidification was 170-4.5 CT (where CR is the cooling rate at ° C./sec and CT is the film thickness (micrometer) of the strip surface. Yes.) We found that the following should be maintained.

In the case of the composition, for example, in an "acid rain" or "contamination" environment and an "acidic" micro environment, the Mg concentration may be 0.5% or more due to the formation of _{Mg 2 Si.}

The Mg concentration may be 1% or more to ensure the effective activity of the alpha phase.

The Mg concentration may be 3% or less. At high concentrations, a coarse, widely dispersed primary Mg ₂ Si phase is formed, which cannot provide uniform activation of the Al-rich alpha phase.

In particular, the Al-Zn-Si-Mg alloy may contain 1% or more of Mg.

Applicants also apply the improved sacrificial protection possible in the present invention to the microstructure range of coarse dendrite structures with an OT: SDAS ratio of 0.5: 1 to fine dendrite structures with an OT: SDAS ratio of 6: 1. I found that.

Applicants also have Al-Zn-Si-Mg alloy coated strips manufactured and subsequently painted as a result of activation of the Al-rich alpha phase and as a result of low levels of cut edges in the marine environment. Shows a narrower and more uniform corrosion front formation.

In the experiments conducted by the applicant, the sample produced by the present invention had a reduced proportion of "edge creep" and "undercutting" from the cut end as compared with the conventional Al / Zn coating.

Improved performance has been demonstrated by applying it to the range of coating structures and the range of paint films.

The present invention will be further described with reference to the accompanying drawings.

The improvement in corrosiveness of the Al-Zn-Si-Mg alloy coated steel strip example according to the present invention has been demonstrated by the applicant in actual "acid rain", "contamination" and test samples exposed in the range of marine environment locations. There is.

The test sample includes a test panel formed by the applicant to provide information about coating corrosion.

FIGS. 1-5 and 1 and 2 show performance improvements of an example of an Al—Zn—Si—Mg alloy coating on a steel strip formed by the present invention in a marine environment.

Performance in the marine environment was evaluated by outdoor exposure test and laboratory cyclic corrosion test (CCT) at ISO rates C2 to C5 according to AS / NZS1580.457.1.1996 Attachment B.

Table 1 shows the painted edge undercut levels of the Al—Zn—Si—Mg alloy coated steel test panel example according to the present invention for the range of metal coating mass (unit: mm) for cleaning exposure in harsh marine environments. The data showing the performance improvement is shown in. The table also includes comparative data with conventional Al / Zn-based alloy coated test panels.

It can be seen from Table 1 that the edge undercutting in the Al-Zn-Si-Mg coated steel panel according to the present invention is very small as compared with the conventional Al / Zn-based alloy coated test panel.

Table 2 shows the range of paint types (units: mm) for wash exposure in harsh marine environments. Edge undercutting of examples of painted Al-Zn-Si-Mg alloy coated steel test panels according to the present invention. The data showing the performance improvement is shown at the level of. The table also includes comparative data with conventional Al / Zn-based alloy coated test panels.

It can be seen from Table 2 that the edge undercutting on the painted Al-Zn-Si-Mg coated steel panel according to the invention is very small compared to the painted conventional Al / Zn-based alloy coated test panel.

The photographs of the test panels and the images of the corroded front in FIGS. 2 to 4 further show the performance improvements of the Al-Zn-Si-Mg coating example according to the present invention in a marine environment. FIG. 2 shows the improvement of the corrosive performance of the fluorocarbon-painted Al-Zn-Si-Mg coating according to the present invention for non-cleaning exposure in a harsh marine environment. FIG. 3 is an example of an extensive corrosion front for a conventional Al / Zn coating under paint in a marine environment. FIG. 4 is an example of a narrower and more uniform corrosion front for the Al-Zn-Si-Mg coating according to the invention under paint in a marine environment.

The photograph of the test panel of FIG. 5 shows the improvement of the corrosion performance of the example of the Al-Zn-Si-Mg coating according to the present invention under the accelerated test conditions. In particular, FIG. 5 shows improvements in surface weather resistance and sacrificial protection of Al-Zn-Si-Mg coatings according to the present invention compared to conventional Al / Zn coatings with coarse or fine structures in salt fog cycle corrosion and testing. show.

6-11 show performance improvements of Al-Zn-Si-Mg coated steel test panels in "acid rain" and "contaminated" environments when manufactured according to the present invention. The photographs show the absence of red rust present on conventional Al / Zn-based alloy coated steel test panels and the absence of red rust on Al-Zn-Si-Mg coated steel test panels manufactured according to the present invention. A comparison of FIGS. 9 and 7 shows that the benefits are retained for extended periods of time. In particular, FIG. 6 shows red rust on conventional Al / Zn-based coated steel strips (total coating mass 100 g / m ^{2) exposed to severe "acid rain" environments for 6 months.} FIG. 7 shows that the Al-Zn-Si-Mg coating according to the present invention (total coating mass of the coating 100 g / m ² ) showed no red rust even after 6 months in the same severe "acid rain" environment. FIG. 8 shows red rust on conventional Al / Zn-based coated steel strips (total coating mass 100 g / m ^{2) exposed to severe "acid rain" environments for 18 months.} FIG. 9 shows that the Al-Zn-Si-Mg coating according to the present invention (total coating mass of the coating 100 g / m ² ) showed no red rust even after 18 months in the same severe "acid rain" environment. FIG. 10 shows that there is red rust on a conventional Al / Zn-based coated steel strip (total coating mass 50 g / m ^{2) with a columnar structure exposed to a severe “acid rain” environment for 4 months.} show. In FIG. 11, the Al-Zn-Si-Mg coating having a columnar structure according to the present invention (total coating mass of the coating: 50 g / m ² ) showed no red rust even after 4 months in the same severe "acid rain" environment. Show that.

Finally, in the microstructure analysis of the Al-Zn-Si-Mg coating example according to the present invention, the applicant has found that the microstructure has a Granular form of Mg ₂ Si phase in a Zn-rich eutectic phase mixed interdendritic channel. It was found that particles were included, which were present between the dendrites of the Al-rich alpha phase, and that this form was important for improving the corrosion resistance of the coating as described above. The applicant _{also found that the size and distribution of Mg 2} Si phase particles are important factors contributing to the improvement of corrosion performance according to the present invention. The applicant _{also found that the morphology, size and distribution of Mg 2} Si phase particles are possible by selecting the coating composition and controlling the cooling rate during film solidification.

12 and 13 show an example of the form of the _{Mg 2 Si phase particles.}

In the planar image of FIG. 12, the dark areas are Al-rich alpha phase dendrites, the bright areas are Zn-rich eutectic phase-mixed interdendrite-like channels, and the "Chinese script" Mg ₂ Si phase particles partially channel. Fulfill.

In the three-dimensional image of FIG. 13, Mg ₂ Si "petals" are shown in red, with other phases including Si (green), MgZn ₂ (blue) and Al-rich alpha phase (dark matrix).

The invention can be modified as long as it does not deviate from the scope and spirit of the invention.

Claims (28)

A method of forming a coating of an anticorrosion Al-Zn-Si-Mg alloy suitable for acid rain, contaminated environments, etc. on a metal strip, typically a steel strip.
(A) A step of passing a metal strip through a melting bath of an Al-Zn-Si-Mg alloy to form an alloy film on both sides or one side of the strip.
(B) Solidify the coating on the strip, extend from the metal strip, contain Al-rich alpha phase dendrites and Zn-rich eutectic phase mixed interdendrite channels, and Mg ₂ Si phase particles in the interdentite channels. The process of forming a solid coating with a microstructure,
The method controls steps (a) and (b) to control the OT: SDAS ratio (in the ratio, OT is the film thickness, and SDAS is the second dendrite arm of the Al-rich alpha phase dendrite of the coating. It is a space.) A method for forming an anticorrosion Al-Zn-Si-Mg alloy film on a metal strip, which comprises forming a solid film having 0.5: 1 or more. The method of claim 1, wherein the OT: SDAS ratio is greater than 1: 1. A metal strip having an Al-Zn-Si-Mg alloy film on one or both sides suitable for acidic rain and contaminated environments, where the film is an Al-rich alpha phase dendrite and Zn-rich eutectic phase mixed inter. It contains a dendrite channel and has a microstructure extending from the metal strip _{and having Mg 2} Si phase particles in the interdendrite-like channel, the coating having an OT: SDAS ratio (in the ratio, OT is the film thickness). , SDAS is the second dendrite arm space of the Al-rich alpha phase dendrite of the coating.) A metal strip having an Al—Zn—Si—Mg alloy coating comprising 0.5: 1 or more. The coated metal strip according to claim 3, wherein the OT: SDAS ratio is greater than 1: 1. When the coating has a total coverage of 200 g / m ² or less on both sides of the strip, it is coated on only one side of the strip and the film thickness is the same as on both sides, then 100 g / m ² or less on the steel strip. The coated metal strip according to claim 3 or 4, which is the same. The coated metal strip according to any one of claims 3 to 5, wherein the film thickness of the coating is 3μ or more. The coated metal strip according to any one of claims 3 to 5, wherein the SDAS of the Al-rich alpha phase dendrite in the coating is larger than 3 μm and smaller than 20 μm. Claim that the Al-Zn-Si-Mg alloy has Al 20-95%, Si up to 5%, Mg up to 10% and the remaining Zn, and a small amount of other elements, typically less than 0.5% each of the other elements. The coated metal strip according to any one of 3 to 7. The coated metal strip according to any one of claims 3 to 8, wherein the metal strip is a steel strip. A method of forming a coating of an anticorrosion Al-Zn-Si-Mg alloy suitable for acid rain, contaminated environments, etc. on a metal strip, typically a steel strip.
(A) A step of passing a metal strip through a melting bath of an Al-Zn-Si-Mg alloy to form an alloy film on both sides or one side of the strip.
(B) The coating is solidified on the strip, extending from the metal strip and containing an Al-rich alpha phase dendrite and a Zn-rich eutectic phase mixed dendrite channel, Mg ₂ Si in the interdented light channel of the solidified coating. A step of forming a solidified film having a microstructure with phase particles,
On a metal strip, the method of selecting Mg and Si concentrations and controlling the cooling rate of step (b) _{to form Mg 2} Si phase particles in an interdendrite channel. A method of forming an anticorrosion Al-Zn-Si-Mg alloy film. The method according to claim 10, wherein the Mg concentration is selected to be 0.5% or more. The method according to claim 10 or 11, wherein the Mg concentration is selected to be 1% or more. The method according to any one of claims 10 to 12, which comprises selecting the Mg concentration to 3% or less. The step of selecting the Mg and Si concentrations and controlling the cooling rate in step (b) is the Mg ₂ Si phase particles in the interdendrite-like channel, which has an appropriate particle size and morphology. The method according to any one of claims 10 to 13, which forms an object that blocks corrosion along. _{The method according to claim 14, wherein the Mg 2} Si phase particles in the interdendrite channel have a "Chinese script" shape when viewed in a plane and a flower petal shape when viewed in a three-dimensional image. The method according to claim 15, wherein the petals have a thickness of 5 μm or less. 15. The method of claim 15, wherein the petals have a thickness of 0.5 to 2.5 μm. The step of selecting the Mg and Si concentrations and controlling the cooling rate in step (b) _{to form the Mg 2} Si phase particles in the interdendrite-like channel is the Al-rich alpha phase in the inter-dendrite-like channel during the solidification coating. 10. The method of any of claims 10-14, wherein the _{Mg 2} Si phase particles are formed in a size range that provides sacrificial protection and has a specific distribution. The cooling rate CR during film solidification is 170-4.5 CT (where CR is the cooling rate ° C./sec and CT is the film thickness μm on the surface of the strip) or less. The method described in either. A metal strip having an Al-Zn-Si-Mg alloy film on one or both sides suitable for acidic rain or contaminated environments, and the film is an inter of Al-rich alpha phase dendrite and Zn-rich eutectic phase mixture. A metal strip containing an Al—Zn—Si—Mg alloy coating, comprising a dendrite channel, extending from the metal strip _{and having a microstructure having Mg 2 Si phase particles in the interdrenated channel.} Claim that the Al-Zn-Si-Mg alloy has Al 20-95%, Si up to 5%, Mg up to 10% and the remaining Zn, and a small amount of other elements, typically less than 0.5% each of the other elements. The coated metal strip according to any one of 3 to 7. The coated metal strip according to claim 21, wherein the Mg concentration is 0.5% or more. The coated metal strip according to claim 21, wherein the Mg concentration is 1% or more. The coated metal strip according to claim 21, wherein the Mg concentration is 3% or less. The coating according to any one of claims 21 to 24, wherein the volume fraction of the _{interdendritic Mg 2} Si phase is 50% or more with respect to the coating having a Si concentration of 0.5 to 2% as compared with other Si-containing phases. Metal strip. The coated metal strip according to any one of claims 21 to 25, wherein the interdendritic _{Mg 2} Si phase has a volume fraction of 80% or more as compared with other Si-containing phases. The coated metal strip according to any one of claims 21 to 26, wherein 70% or more of the total integral ratio of the Mg _{2 Si phase of the coating is 2/3 or less of the coating thickness of the coating.} The coated metal strip according to any one of claims 21 to 27, wherein 60% or more of the interdendritic channels are _{blocked by Mg 2 Si phase particles.}

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